Using forensic microsatellites to decipher the genetic structure of linguistic and geographic isolates: A survey in the eastern Italian Alps

Abstract

The study of geographically and/or linguistically isolated populations could represent a potential area of interaction between population and forensic genetics. These investigations may be useful to evaluate the suitability of loci which have been selected using forensic criteria for bio-anthropological studies. At the same time, they give us an opportunity to evaluate the efficiency of forensic tools for parentage testing in groups with peculiar allele frequency profiles. Within the frame of a long-term project concerning Italian linguistic isolates, we studied 15 microsatellite loci (Identifiler kit) comprising the CODIS panel in 11 populations from the north-eastern Italian Alps (Veneto, Trentino and Friuli Venezia Giulia regions). All our analyses of inter-population differentiation highlight the genetic distinctiveness of most Alpine populations comparing them either to each other or with large and non-isolated Italian populations. Interestingly, we brought to light some aspects of population genetic structure which cannot be detected using unilinear polymorphisms. In fact, the analysis of genotypic disequilibrium between loci detected signals of population substructure when all the individuals of Alpine populations are pooled in a single group. Furthermore, despite the relatively low number of loci analyzed, genetic differentiation among Alpine populations was detected at individual level using a Bayesian method to cluster multilocus genotypes. Among the various populations studied, the four linguistic minorities (Fassa Valley, Luserna, Sappada and Sauris) showed the most pronounced diversity and signatures of a peculiar genetic ancestry. Finally, we show that database replacement may affect estimates of probability of paternity even when the local database is replaced by another based on populations which share a common genetic background but which differ in their demographic history. These findings point to the importance of considering the demographic and cultural profile of populations in forensic applications, even in a context of substantial genetic homogeneity such as that of European populations.

Introduction

Autosomal microsatellites are widely used in forensics because of their relative ease of determination and high discriminatory power among individuals, properties which make these loci valuable tools for both parentage testing and personal identification [1]. While these loci represent excellent markers of genetic identity, their high mutation rate enables microsatellites to detect differentiation among closely related populations or, even, subpopulations [2]; but see [3]. However, as a side effect, this property may be detrimental to the robustness of forensic parameter estimates when populations under study are genetically sub-structured or a population-specific database is not available.

The autosomal microsatellites included in the Combined DNA Index System (CODIS) may be regarded as an important reference for any laboratory working in forensic genetics [http://www.fbi.gov/about-us/lab/codis]. Initially set up by the Federal Bureau of Investigation (FBI) in the US, the CODIS panel has rapidly become a widespread standard worldwide. In fact, CODIS has been characterized in a number of European populations [4], [5], [6]. On the whole, a substantial homogeneity has been observed among large and open European populations, with Fst as low as 0.0028 [7]. However, only a few studies have regarded small sized populations which are subject to potential isolation factors. These investigations may be useful to evaluate the suitability of microsatellites which were originally designed for forensic applications in bio-anthropological studies. Furthermore, they can give us an opportunity to evaluate the forensic efficiency of CODIS loci in populations where cultural diversity and geographical factors may constrain gene flow, leading to peculiar profiles of allele frequencies [8], which would, in effect, challenge the claimed genetic uniformity of European populations [9].

Within the frame of a long-term project on Italian linguistic and geographic isolates [10], we genotyped 15 microsatellite loci including the CODIS panel in 11 populations from the north-eastern Italian Alps (Veneto, Trentino and Friuli Venezia Giulia regions). Coherently with the premise, this investigation focuses on both bio-anthropological and forensic issues. The data obtained are first analyzed using a variety of population genetic tools in order to understand how microsatellite variation is structured in a broad sampling of populations from an area with important physical barriers to human mobility and substantial linguistic diversity. Thereafter, we evaluate the performance of the CODIS panel for paternity testing with the aim of assessing the impact of potential isolation factors on the robustness of forensic estimates.